The invention relates to an infrared modulator for spectrometer, which comprises a light source, a beam splitter for splitting the light from the source into two beams, a first plane mirror for directing the light of the first beam, a second plane mirror for directing the light of the second beam, a first cube corner mirror formed of three mirrors perpendicular to each other for turning the light directed by the first plane mirror back to the first plane mirror, and a second cube corner mirror formed of three mirrors perpendicular to each other for turning the light directed by the second plane mirror back to the second plane mirror, the first and second cube corner mirrors being arranged on a common optic axis to reflect into opposite directions and movable back and forth in the direction of said optic axis. The mirror system formed by the cube corner mirrors is a double cube corner whose task is to return the light directed to it exactly to its incoming direction to the above-mentioned first and second mirrors and through them to the beam splitter in which the light beams interfere with each other.
The invention is intended for use especially in a Fourier transformation spectrometer of the infrared or near-infrared regions to modulate radiation into a format that allows the calculation of the spectral distribution of the radiation to be measured by using Fourier transformation.
When the path lengths of the two light beams are exactly the same, an interference maximum of all wavelengths is detected in the output of the device. When moving the double cube corner in the direction of the light beams coming from the first and second mirrors, the wavelength distribution of the light passing through the device can be measured utilizing the interferences of different wavelengths.
In Fourier transformation infrared (FTIR) spectroscopy, many kinds of devices are used in modulating infrared radiation, the simplest being the Michelson interferometer based on the use of plane mirrors. In spectroscopic applications, it is very important that the movement of the mirror generating modulation does not cause changes in the alignment of the beams. This problem has been solved for instance by using what is known as the dynamic alignment system, in which the alignment of the mirror of one of the beams of the interferometer is changed continuously so as to maintain the modulation unchanged. Attempts have also been made to change the movement of the mirrors such that it does not cause changes in modulation. This has been attempted for instance by using a rotational movement instead of a linear movement.
The problems of alignment caused by plane mirrors have also been solved by replacing the plane mirrors with cube corner mirrors in the Michelson interferometer, but these have not been able to achieve a sufficiently stable structure for field conditions. The double-beam interferometers described in U.S. Pat. Nos. 4,165,183 and 4,319,843 are also based on the use of cube corner mirrors, but neither of them is designed for Fourier transformation spectroscopic applications and, thus, their solutions for the moving mechanism, for instance, do not serve the purpose of the present case.
A focusing interferometer, as known for instance from U.S. Pat. No. 5,459,572, has nearly achieved the desired field usability, but the solution has, in practice, proven sensitive to vibration, because the spherical mirrors used as end mirrors are unavoidably massive and, thus, susceptible to mechanical disturbance.